1. Field of the Invention
This invention relates to an improved vapor phase oxidation process for the preparation of maleic anhydride from n-butane over a catalyst containing mixed oxides of vanadium and phosphorus.
2. Background
An important route to maleic anhydride is the vapor phase oxidation of n-butane over a vanadium/phosphorus oxide catalyst. The reaction step involves oxidation of n-butane with air (oxygen) to form maleic anhydride, carbon oxides, water and smaller amounts of partially oxidized by-products. Typically, the process is carried out in multitubular fixed bed reactors. The large exothermal heat and the thermal sensitivity of the n-butane oxidation requires low feed concentrations, expensive heat-transfer equipment, handling of a large volume of gas, and good reactor temperature control. Low butane concentration is also required to avoid flammability conditions. The magnitude of some of these problems is reduced when a fluidized bed reactor is used. The temperature can be readily controlled within a few degrees because of the intensive catalyst mixing and good heat transfer characteristics. Higher butane concentrations can be used because the danger of flammability is greatly reduced by introducing the n-butane directly into the reactor rather than premixing it with air (oxygen). However, very high butane concentrations and low oxygen-to-butane ratios in the reactor may result in over reduction of the catalyst and reduced selectivity to maleic anhydride. Also, significant back-mixing of gases in the fluidized bed reactor encourages maleic anhydride oxidation reactions which reduce selectivity. In addition, rapidly rising gas bubbles in a fluidized bed reactor result in poorer contact between gases in the bubbles and the catalyst, making it difficult to achieve high butane conversion.
A recirculating solids reactor has two reaction zones, in which two separate reactions take place, and a catalyst (the solid) which circulates between the two reaction zones and takes part in both reactions. Such reactor systems have found use in catalystic cracking in petroleum refining and in other reactions. U.S. Pat. No. 4,102,914 discloses a process for the preparation of acrylonitrile by passing a gaseous mixture comprising propylene, ammonia and molecular oxygen and an ammoxidation catalyst through a reaction zone while controlling the superficial linear gas velocity and solids feeds rate to achieve a state of fast fluidization. In a preferred embodiment the lower part of the reactor serves as a regeneration zone and recycled catalyst from the separator is contacted with molecular oxygen prior to the addition of ammonia and propylene. U.S. Pat. No. 4,261,899 discloses a process for preparing phthalic anhydride by oxidizing o-xylene with air (oxygen) in a dilute phase transported bed reactor. Substantially all of the o-xylene is introduced at one end of the reactor while oxygen necessary for the reaction and fluidized catalyst are introduced at a plurality of spaced inlets along the reactor. The catalyst is separated from the product gases and recycled. European Patent Office Publication No. 0 034 442 discloses a process for preparing unsaturated aldehydes (or unsaturated acids) by passing an unsaturated olefin (or unsaturated aldehyde) and oxygen into a transport line reactor with a solid oxidation catalyst to achieve substantially plug flow within the reactor. Reaction products are stripped from the catalyst with steam in the stripper chamber.
The preparation of mixed oxide compositions of vanadium and phosphorus and the use of these as catalysts for the oxidation of hydrocarbons to maleic anhydride is known in the art. In U.S. Pat. Ser. No. B 330,354 and U.S. Pat. No. 4,111,963 the importance of reducing the vanadium used in a vanadium/phosphorus/oxygen (V/P/O) catalyst to the +4 oxidation state is described. Preferred is the use of concentrated hydrochloric acid as the reaction medium to bring about this reduction and preferred catalysts have a phosphorus to vanadium atom ratio of 1:2 to 2:1 and a porosity of at least 35%. In U.S. Pat. No. 3,864,280 the reduction of the vanadium in such a catalyst system to an average valence state of 3.9 to 4.6 is emphasized; the atomic ratio of phosphorus to vanadium is 0.9 to 1.8:1. Isobutyl alcohol is used as a solvent for the catalyst preparation, with the indication that an increase in catalyst surface area, over that obtained from use of an aqueous system, is achieved. The addition of promoters to the vanadium/phosphorus oxide catalyst compositions used for the oxidation of hydrocarbons to maleic anhydride is also disclosed in the art. Thus, in U.S. Pat. No. 4,062,873 and U.S. Pat. No. 4,064,070 are disclosed vanadium/phosphorus/silicon catalyst compositions made in an organic medium. In U.S. Pat. No. 4,132,670 and U.S. Pat. No. 4,187,235 are disclosed processes for preparing high surface area vanadium/phosphorus oxide catalysts. Anhydrous alcohols of 1-10 carbon atoms and 1-3 hydroxyl groups are used to reduce the vanadium to a valence of 4.0 to 4.6. Also disclosed are such catalysts containing up to 0.2 mol, per mol of vanadium, of a transition, alkali or alkaline earth metal, for example, tantalum, titanium, niobium, antimony, bismuth or chromium. In U.S. Pat. No. 4,371,702 is disclosed an improved vanadium/phosphorus oxide catalyst containing the promoter comprising silicon and at least one of indium, antimony and tantalum. Zazhigalov et al., React. Catal. Lett. 24(3-4), 375 (1984), report on microcatalytic studies of the catalytic oxidation of butane over a V/P/O catalyst and have concluded that maleic anhydride forms mainly due to the gas phase oxygen. They state that on this catalyst the rate of maleic anhydride formation and the process selectivity in the presence of gas phase oxygen is significantly higher than in its absence and that, therefore, only a small part of the maleic anhydride can form at the expense of the catalyst oxygen. C. N. Satterfiled, Heterogeneous Catalysis in Practice, McGraw-Hill Book Company, 1980, page 186, discloses that vanadium and other metal oxides capable of rapid and reversible oxidation-reduction may be used as an oxygen carrier to cause a patial oxidation reaction, with the carrier being reoxidized in a separate reactor. D. J. Hucknall, Selective Oxidation of Hydrocarbons, Academic Press, 1974, page 36, discloses that selective alkene oxidation over a bismuth molybdate catalyst may be carried out either in the presence or absence of gaseous oxygen.
The following U.S. patents are relevant with respect to a process for the vapor phase oxidation of n-butane to maleic anhydride over a vanadium/phosphorus oxide catalyst, wherein such process the ratio of n-butane to oxygen is greater than the most commonly employed ratio of about 1:10 and the less commonly employed ratio of 1:4. However, in all of these patents the disclosed procedures require more oxygen in the feed gas than the stoichiometric amount required to convert the total amount of n-butane converted in the process to maleic anhydride. None discloses isolating the reduced catalyst and reoxidizing it before contacting it again with n-butane, none discloses operating with the feed gas substantially free of oxygen, and none discloses operating with the feed gas substantially free of oxygen and the reoxidized catalyst being stripped of oxygen before being contacted again with n-butane.
U.S. Pat. No. 3,899,516 discloses an improved process wherein the oxidant is substantially pure oxygen, rather than oxygen diluted with inert gases, for example, with nitrogen, as in air, and the ratio of butane to oxygen is greater than 1:4. Preferably, the ratio is in the range of about 1:1 to 20:1. Mole ratios of 1.6:1 and 3:1 are exemplified but the conversions are 2.2% or less and 1.14%, respectively, so that the amount of oxygen present is more than 8 times the stoichiometric amount required to convert the total amount of n-butane converted in the process.
U.S. Pat. No. 3,904,652 discloses an improved process comprising maintaining, in the reaction zone, an n-butane concentration above 1.7% of the feed, an oxygen concentration of 3-13%, and an inert gas concentration of 70-95%; converting from 30 to 70% of the n-butane; withdrawing a reactor effluent; separating the maleic anhydride; and recycling a major portion of the remaining reactor effluent to the reaction zone. In the five runs disclosed in the example, the amount of oxygen present in at least twice the stoichiometric amount required to convert the total amount of n-butane converted in the process.
U.S. Pat. No. 4,044,027 discloses, in an example, a process wherein the feed gas stream contains 9.1% oxygen and 6.2% butane; however, only 25% of the n-butane is converted.
U.S. Pat. Nos. 4,151,116 and 4,244,878 disclose the use of a V/P/O catalyst having a promoter which is post-deposited upon its surface. The patents further disclose that, typically, the oxidation of butane to maleic anhydride is carried out by means of air or other molecular oxygen-containing gases such as mixtures of carbon dioxide and oxygen or mixtures of nitrogen or steam with air or oxygen, that air is preferred, and that preferably the concentration of butane in the feed will be 1.0 to 1.6 volume % with the oxygen above 10 volume %, and 1.5 to 5 volume % with the oxygen below 0.1 volume %. The examples disclose only the use of a mixture of 1.5 volume % of n-butane in air.
U.S. Pat. No. 4,222,945 discloses a process for oxidizing 10 to 50% of a linear C.sub.4 hydrocarbon by contacting it with oxygen. In the process the hydrocarbon concentration is at least 10 molar % and is higher than the explosive limit, the oxygen concentration is greater than 13 molar % of the total material fed to the reaction, and the concentration of any inert gas present is less than 70 molar % of the total material fed to the reaction. The preferred oxygen concentration is 14 to 30 molar %. In all of the examples, the amount of oxygen present is in excess of the stoichiometric amount required to convert the total amount of n-butane converted to maleic anhydride in the process. U.S. Pat. No. 4,317,777 discloses a process for oxidizing 10 to 50% of a hydrocarbon which comprises at least four linear carbon atoms by contacting it with oxygen, the hydrocarbon concentration being greater than 10 molar % and being higher than the explosive limit, the oxygen concentration being greater than 13 molar %, and the concentration of inert gas being greater than 70 molar % of the total material fed to the reaction. The preferred oxygen concentration is 14 to 20 molar %. In the examples, the amount of oxygen present is in excess of the stoichiometric amount required to convert the total amount of n-butane converted to maleic anhydride in the process.
U.S. Pat. No. 4,342,699 discloses a process comprising contacting a non-flammable, n-butane-rich feed consisting essentially of about 2 to about 8 mole % n-butane, about 8 to about 20 mole % molecular oxygen, and a balance of at least one inert gas, and an n-butane oxidation catalyst, in a heat transfer medium-cooled, tubular reaction zone maintained under oxidation conditions which are such as to yield a relatively low per pass conversion of n-butane, the catalyst being graded along at least a portion of the effective length of the reaction zone so as to provide minimum reactivity nearest the feed end of the reaction zone and maximum reactivity nearest the exit end of the reaction zone, and recycling a portion of the effluent with addition of make-up gases comprising n-butane and oxygen.
In all of the examples, the amount of oxygen present is in excess of the stoichiometric amount required to convert the total amount of n-butane converted to maleic anhydride in the process.
U.S. Pat. No. 4,352,755 discloses a process for producing a gas stream containing at least 2.5% by volume maleic anhydride by oxidizing a straight chain C.sub.4 hydrocarbon with oxygen. The feed stream comprises: the C.sub.4 hydrocarbon (25% to 60%, preferably 40 to 55%, and more preferably 45 to 50% by volume); oxygen (20 to 45%, preferably 25 to 35% by volume); and, optionally, inert gases (0 to 30%, preferably 5 to 25%, and more preferably 12 to 20% by volume).
An object of this invention is to provide an improved vapor phase oxidation process for the preparation of maleic anhydride from n-butane over a catalyst containing mixed oxides of vanadium and phosphorus. A further object is to provide a vapor phase oxidation process for the preparation of maleic anhydride from n-butane over a catalyst containing mixed oxides of vanadium and phosphorus, which process is carried out in a recirculating solids reactor. Recirculating solids, transported bed and riser reactors are well known in the art, as is evident from Kahney et al., 66th Annual Meeting, American Institute of Chemical Engineers, Philadephia, Pa., November, 1973; U.S. Pat. No. 3,799,868; and Robertson et al., Chemical Science, 36, 471 (1981). Other objects will become apparent hereinafter.